The present invention concerns a method in a paper manufacturing process for improving the properties of paper, in particular the retention of, e.g., fines and fillers, in which the fibre material is passed through a refining and screening system to a head-box arrangement of a paper machine having at least one headbox, and through which the stock suspension is fed onto a forming wire or into a forming gap between forming wires.
The principal raw-material for paper comprises fibres derived from cell tissues of various plants. The most important fibres for the manufacture of paper are obtained from softwood or hardwood, however fibres obtained from, e.g., straw or bagasse can be utilized in certain cases. The fibres can be separated from the wood raw material by a chemical or mechanical defiberizing process. Such defiberizing processes are known in the art in and of themselves, so that detailed description thereof is not necessary for an understanding of the present invention. The fibre material obtained by the chemical method is generally called chemical pulp, while the fibre material produced mechanically is usually called groundwood pulp or mechanical pulp. There are also intermediate forms between these manufacturing proesses, with pulp types obtained by such means or intermediate forms.
In the paper manufacturing process, the fibres are first pre-treated in a suitable manner, principally mechanically in particular refiners. Thereupon, the fibres are suspended into a 100-fold to 200-fold quantity of water, to form a fibre-water suspension, which is then passed to the paper machine.
An important difference between the chemical and mechanical pulps, is that the mechanical pulps can be passed practically directly from the defiberizing stage to the paper machine, although fibres of chemical pulp must be processed in a certain manner and treated by refining before paper can be manufactured from the same. Moreover, due to the process of manufacture, the average fibre length of mechanical pulp, is, as a rule, shorter than that of chemical pulp. However, the average fibre length in a pulp depends primarily upon the raw-material of pulp itself.
By nature, the average length of softwood fibre is most usually about three-fold, as compared to hardwood fibre. It has also been ascertained that fibre obtained from different hardwood species, may differ from one another, e.g., with respect to the length thereof, and also with respect to other properties, differing even tremendously in this regard. Extreme examples in this respect are birch fibres on the one hand, and eucalyptus fibres on the other hand, both of which are commercially and industrially important raw materials for the paper industry.
The principal parts of a paper machine are the head box, the forming section, the press section, and the drying section. The paper is manufactured in a continuous process, so that the fibre suspension is fed through the head box as an even layer onto an endless wire fabric moving forwardly and included in the web former, where the suspension water drains principally through the meshes in the wire fabric while the fibres are intertwined and bound together to form an integrated wet fibre mat or web. Further removal of water out of the web formed in this manner described above takes place mechanically in the press section of the paper machine and in the drying section by means of heat evaporation, whereby a completely dry paper web is obtained as the final result.
The ultimate properties of the paper produced are determined, in addition to the particular type of raw-material used, by the manner in which the paper machine and the webforming process treat these particular raw-materials. The most important operative factors in the mechanism of forming the paper web, are the head box and the wire part whose cooperation is decisively important for the web formation.
The fibre material or stock from which the paper is manufactured, is by nature highly nonhomogeneous with respect to both the length and the thickness of the fibres. The longest fibres are of an order of 2 to 3 mm, while the shortest fibres are about 1/10 of this length. Moreover, the fibre stock contains varying quantities of so-called tines, i.e. indefinite fragments of fibre. As a matter of fact however, this non-homogeneity of the stock is necessary for the formation of even paper. With simplification, it can be said that the long fibres contained in the stock form the fibre network constituting the basic structure of the paper web, while the rest of the fibre material fills the meshes in this network.
Only few paper qualities are produced by using a single fibre type alone. Such qualities are, e.g., kraft papers, which are, as a rule, manufactured from softwood pulp only. In most cases, at least two kinds of fibre are used for paper, such as, e.g., for newsprint, whose fibre composition may comprise 75 to 85 percent mechanical pulp and correspondingly, 15 to 25 percent chemical pulp. The reason for the use of different types of fibres may be exclusively a matter of cost. As a rule, attempts are made to use as inexpensive fibres as possible, if the quality requirements imposed on the paper permit it.
Recently, the paper industry has encountered several serious problems. The cost of cellulosic pulp has increased. Authorities have imposed even stricter ecological requirements which have increased the cost of paper manufacture. Also, the general direction of evolving energy cost of paper manufacture, has been increasing. These circumstances have placed the paper industry and its customers in a situation of having to make a choice. Either the higher costs must be paid for, or the proportion of cellulose fibres must be reduced or fibres of inferior quality must be utilized, which, with the present paper manufacturing techniques, results in a deterioration of the quality of the paper products, in particular of the printing properties thereof.
However, in most cases the reason for the use of fibre mixtures, is that certain fibres give the paper desirable properties. Thus, some fibres give the paper increased strength, while other fibre types may improve other properties, e.g. brightness, smoothness, opacity, or porosity. There are numerous fibre combinations, and also combinations of properties which are desired in paper.
The exclusive role of fibre material in paper manufacture and as a factor affecting the properties of paper, has been dealt with above. However, several paper qualities, in particular those intended for printing, contain considerable amounts of mineral so-called fillers or pigments. Moreover, coloring agents, sizes or other chemicals may be added to paper stock in the amount of a few percent. The proportion of the latter additives in the stock is however, small as compared, e.g., with the fillers, whose proportion is most generally 10 to 40 percent of the weight of finished paper. By means of the fillers, a number of good properties are obtained in the paper, of which some of the most important ones are opacity, smoothness and glazability of the paper, as well as, moreover, elasticity of the paper which reduces, e.g., rustling of the paper.
In accordance with the above, the paper comprises several components, with the main groups thereof being the fibres proper, the tines, and the fillers. When the relative proportions of these components are appropriately chosen, the desired properties can be obtained in the finished paper. The web forming process on the wire part and the constructional factors of the wire part affecting the process may cause an amount of valuable components, above all fine fibres and fillers, to be lost from the stock along with water being removed, at the same time that the water is being removed out of the fibre suspension. The property of the inchoating web that is in this stage on the wire part, for retaining the fine fibres and filler particles present in the fibre suspension in the paper structure during the dewatering process, is termed the ability of retention. Besides the fibre material itself and its, e.g. physical-chemical properties, the retention is also affected by many external factors, such as mesh of the wire fabric, type of the forming members, running speed of the paper machine, etc. The web forming process must be arranged to take place and must be controlled so that in particular, the content of fines in the stock and in the finished paper is as high as possible, and that the distribution in the direction of thickness of the paper is assymmetric as possible. In particular, paper intended for printing must not show so-called two-sidedness, but both sides must be as equal as possible regarding smoothness and porosity, i.e. printing properties thereof.
Such web-forming processes, by means of which these objectives of quality of paper are achieved, can best be accomplished by means of so-called twin-wire formers, of which there are several different types.
One such twin-wire former is described in Finnish Patent Application No. 842050. It is an essential feature of the operation of this twin-wire former that at the initial stage of the web-forming, the dewatering arranged to take place very gently in two directions. In this manner, a pre-couched fibre layer is initially obtained on both wires, such layers acting as filters for the fines contained in the pulp suspension as the dewatering continues and is gradually intensified. In contrast thereto, in the most common twin-wire formers the dewatering takes place right from the start extremely violently, which is not advantageous for the paper forming process.